This invention relates to the art of alloys for dental purposes and the like, and more particularly to a new and improved alloy for use with porcelain for dental restorations.
There have been several generations of alloys for use with porcelain for dental restorations, and the first successful alloy system, known as yellow alloy, had a relatively high gold content, i.e. about 85 percent by weight, with platinum and palladium being the primary additions. The second generation alloy, known as white alloy, included a greater proportion by weight of platinum to provide greater strength. Eventually, cost reduction became an important consideration, and as a result both the yellow and white alloys were diluted with silver and the high priced platinum was removed. Then gold was removed to provide the final cost reduction.
The addition of silver, however, gave rise to an unexpected and undesirable result. Dental castings frequently are preoxidized in a temperature range of 1700.degree. F. to 1950.degree. F. prior to application of porcelain on the metal with the resulting oxide acting as a bonding agent between the metal and porcelain. Also, it is common to put a gold layer on the metal, before porcelainizing, by painting a gold powder slurry on the metal and then melting the gold in place at a temperature of about 1945.degree. F. The porcelain then is fired to the surface of the metal in several "bakes" up to a temperature of about 1750.degree. F.-1850.degree. F. Thus, the cast substrate is in the furnace several times at temperatures ranging from about 1200.degree. F. to about 1950.degree. F.
Silver has a vapor pressure of about 1 .mu.m at 1700.degree. F. and about 10 .mu.m at about 1950.degree. F. and consequently some silver can vaporize during firing operations if the concentration in the alloy is sufficiently high. The resulting silver vapor then may condense in a very finely divided state on the surface of the porcelain being fired. If another layer of porcelain is fired over this condensed silver, the silver acts as a pigment and changes the shade of the porcelain. The most noticeable change is the grey of the silver interacting with the yellow tint of the porcelain to produce a greenish hue which is very undesirable.